What Berlin is Building is Not What It Needs to Build

Berlin has a deceptively simple S-Bahn network. There’s the Ringbahn circling city center. There’s the elevated east-west Stadtbahn, which has two tracks dedicated to S-Bahn service and two for everything else, including longer-range regional trains and intercity trains. And there’s the two-track North-South Tunnel, which only carries S-Bahn traffic; longer-range traffic uses the four-track north-south mainline through Berlin Hauptbahnhof, whereas the North-South Tunnel intersects the Stadtbahn one station east of Hauptbahnhof, at Friedrichstrasse.

The main S-Bahn capacity needs in Berlin are east-west; meanwhile, the North-South mainline is underfull, with Wikipedia listing around 7 trains per hour. And yet, Berlin’s big S-Bahn capital project is a new tunnel, dubbed S21, adding yet another north-south track pair through Hauptbahnhof. Fortunately, the project is salvageable, but only if the city and the federal government act quickly, within a few years, to change yet-unbuilt phases to run in the right direction.

Berlin urban rail traffic map

Here is a map of traffic demand on every interstation on the combined Berlin U- and S-Bahn network (source, p. 6):

The numbers are in thousands of passengers per weekday in both directions combined.

The U-Bahn is in blue. It’s a weird-looking network because two lines (U7, running northwest-southeast in the west, and U9, running north-south also in the west) were built in the Cold War to serve West Berlin’s center around Kurfürstendamm, whereas the S-Bahn and the older U-Bahn lines serve the historic center. Since reunification, Germany has made an effort to move the Berlin central business district back to the historic center, and S21 is to reinforce that, serving the western end of Mitte.

Unfortunately, as we see in the green lines, that’s not where the pressing S-Bahn capacity needs are. First, the Stadtbahn is busier than the North-South Tunnel. Second, the busiest branches heading into the city come from the east, with substantially more traffic than from the north and south.

And then there’s the Görlitz Railway. It is the line heading to the southeast, without its own trunk line through the city – it reverse-branches to the two directions of the Ringbahn. Moreover, going north there’s additional reverse-branching, to the Stadtbahn (S9) and around the Ring to the northern branches (S8, S85), with each service running only every 20 minutes. Total traffic across these services is quite high, 107,000 weekday passengers, compared with 144,000 on the Prussian Eastern Railway (S5, S7, S75; S5 is the mainline), 128,000 between the two branches feeding the North-South Tunnel from the south, and 133,000 between the two branches feeding the North-South Tunnel from the north. The brief segment where S9 runs alongside the Ring has 184,000 weekday passengers, the city’s busiest.

S21: what Berlin is actually building

Berlin Hauptbahnhof is a new station. It only opened in 2006, when the North-South Intercity Line opened. The new four-track line has ample capacity for additional S-Bahn traffic, but nonetheless it hosts no S-Bahn trains in regular service. Instead, there are plans for two additional S-Bahn tracks, mostly in tunnel, parallel to the line, with service to Hauptbahnhof:

The map does not show the phasing. The segment from the Ringbahn in the north down to Hauptbahnhof is just about complete, with opening expected soon. The segment from Hauptbahnhof to Potsdamer Platz, which contrary to the map is to be nonstop, is in early stages of construction, and Wikipedia says it is expected to open in 2023.

Farther south of Potsdamer Platz is still not under construction, and frankly should not be built as is. The only real addition this would give to the network is the stop at Gleisdreieck, where the line intersects the east-west U1; the North-South Tunnel intersects U1 without a connection, the only place in the city where there is a missed U-Bahn/S-Bahn connection unless one counts the marginal U9/Stadtbahn miss in which the next station, Zoologischer Garten, is a transfer.

However, the North-South Main Line’s tunnel portal lies just south of Gleisdreieck, and thus it should be feasible if nontrivial to add platforms there for two of the tracks. Farther south, at Yorckstrasse, it is well outside the portal and adding platforms should be fairly easy.

Görlitz Railway S-Bahn: what Berlin should be building

A radial rail network with three lines should aim to have them meet at a triangle in city center. Berlin has two S-Bahn radial lines, and S21 is to add a third. Instead of running parallel to the North-South Tunnel, it should provide a third trunk line. North of Potsdamer Platz the route is already baked in, but farther south, the Görlitz Railway route is a perfect legacy line to link to. It is quite busy, and the likely locations of the intermediate stops between existing infrastructure and Potsdamer Platz are busy U-Bahn stations in their own right.

I was delighted to see this already discussed on the technical transit blog Zukunft Mobilität. It has a long list of potential Berlin rail extensions, some in accordance with current long-term plans, some not. It specifically criticizes S21 for duplicating existing infrastructure, and proposes an extension to the southeast, mentioning that there were plans to that effect in the 1930s. There are two variants, one through Hermannplatz and one through the old route of the Görlitz Railway.

The dashed lines denote under-construction lines, including S21 to Potsdamer Platz, the 4.5-kilometer Siemens Railway to the northwest, and the U5-U55 connection. Dotted lines denote lines I am proposing: either variant connecting S21 toward the southeast, paired with the Siemens Railway as well as two new-build lines through the area of Tegel Airport, which is slated for redevelopment after the Berlin-Brandenburg Airport finally opens. Two branches are depicted toward Tegel, one toward airport grounds to be redeveloped, and one going farther north taking over S25; there are already discussions of a rapid transit line to Tegel, branching off of U6, but this option does not force the outer parts of U6 to contend with reduced frequency.

The two branches should of course not both be built. The main advantage of the southern option is that it hits Hermannplatz, one of the busiest stations in the system: the above diagram of rail ridership shows a large change in U8 demand north and south of the station, and a factsheet from 2010 asserts that it is the second busiest U-Bahn station, closely behind Alexanderplatz. In effect, it functions as an express link from Neukölln to city center. U8 isn’t especially crowded – nothing in Berlin is – but it’s busiest than the North-South Tunnel; this link is at least as justified as the S21 tunnel to the south. This would require about 7 km of tunnel. While S-Bahn tunnels cost more than U-Bahn tunnels, this is deliberately an express line, so keeping costs down to the per-km level of the U5-U55 connection (€525 million for 2.2 km) is reasonable, making it a €1.8 billion project or thereabout.

The northern option works differently. It doesn’t hit anything as interesting as Hermannplatz on the way, but it does serve Alt-Treptow, one of the bigger rapid transit deserts inside the Ring. The infill station would also break what is the second or third longest interstation on the Ring. Closer in, it has better coverage in the center – Checkpoint Charlie offers another CBD station in addition to Potsdamer Platz. The cost is more of an open question here. From Görlitzer Bahnhof to Potsdamer Platz it’s about 4 km; east of Görlitzer Bahnhof it’s plausible that the line could reuse the Görlitz Railway’s right-of-way and run elevated, or at worst underground with cut-and-cover. However, the per km cost of the tunnel would be higher, since proportionately more of it is in city center, and it has the same number of stations over shorter length; my vague guess is somewhat less than €1.5 billion.

The Berlin S-Bahn would become a system with three radial lines, meeting at Hauptbahnhof, Friedrichstrasse, and Potsdamer Platz. All reverse-branching would cease: the various branches on the Görlitz Railway, including the existing ones as well as an under-construction one to the airport-to-be, would feed into the S-Bahn trunk, rather than to the Ring or the Stadtbahn. The removal of S25 from the North-South Tunnel would create space for the S8 and S85 services in Pankow to use the North-South Tunnel instead of diverting to the Ring and Görlitz Railway. Potentially, the North-South Tunnel could also be realigned to serve Gleisdreieck, as depicted on the map. Finally, with S9 removed from the Stadtbahn, there would be room to beef up service on S3 and/or end the current practice in which S75 trains from the east stop at Ostbahnhof rather than running through.

Germany isn’t perfect

Writing about North America, I talk a lot about how it can Germanize its regional rail network. But it’s important to understand that while far better than North America, Germany is not perfect. It makes mistakes of many kinds: some involving high construction costs, some involving schedule slips, some involving unnecessary prestige projects. These can mostly be prefaced by “by Continental standards,” though the Berlin-Brandenburg Airport disaster is bad even by the standards of the Anglosphere and its billion-dollars-per-kilometer subways.

The Berlin S-Bahn is a case in point. It has a pretty hefty peak-to-base ratio by German standards – the Ring lines (S41 and S42) run every 5 minutes peak and every 10 off-peak, and a number of other lines have a peak-to-base ratio of 2 as well. It also has a peculiarity in that S75 trains only run east of Ostbahnhof; I can’t tell if there’s a problem with track capacity or demand mismatch, but if it’s the former then it’s strange since peak S-Bahn traffic on the Stadtbahn is only 18 trains per hour (Munich achieves 30 through its central tunnel, with much higher crowding levels), and if it’s the latter then it’s again strange – why not run through to Westkreuz like S5?

S21 is another of these little mistakes. It’s a prestige project on the heels of the construction of Hauptbahnhof, rather than a solution to a transportation need. There are six north-south tracks through Berlin between the S-Bahn and the mainline and they’re not anywhere near capacity; the mind boggles at why anyone would add seventh and eighth tracks before adding fifth and sixth east-west tracks.

Fortunately, the mistake is fixable. Germany’s dragging infrastructure timeline means that there’s often room for modifications to make things more useful. The airport is a lost cause, but S21 is not. From Potsdamer Platz south there’s a good option that adds S-Bahn service exactly where it is needed and simplifies citywide schedules by making it feasible to eliminate reverse-branching. In lieu of building more autobahns, Berlin should commit to building the southeastern extension via Alt-Treptow or Neukölln.

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41 comments

S75 ending at Ostbahnhof is said to be a matter of capacity on the central part of Stadtbahn. I can’t speak for Munich – are their stations bigger with better circulation, perhaps? – but having seen the central Stadtbahn stations in practice it would be quite a challenge to increase service to them by 67%.

I want to say something about reverse-branching, but Munich does have that on the S-Bahn, just less than Berlin (whereas its U-Bahn is the most reverse-branched I know of outside New York, even more than London and Washington).

I don’t think Berlin should even aim at 30 tph on the Stadtbahn. But getting up to 24 should be doable; Paris achieves that with way worse peak crowding.

RER. The RER stations aren’t necessarily better when it comes to circulation, e.g. Les Halles has very wide platforms but not a lot of exits for how high the ridership is, leading to queues at the bottoms of the escalators.

Surprised that the U5 Hauptbahnhof-Tegel extension is not on your crayon map (along with keeping Tegel open!) The Moabit area is pretty dense and extending the U5 (instead of the U6) eliminates one of the most obvious missing radials to the reunified center of Berlin and growth in the East.

The second most obvious missing Ubahn radial is to the northeast between the U2 and U5. It seems like one of the U1/U3/U4 could be extended from Schöneberg through Alexanderplatz and out to at least the Weissensee area if not Wartenburg. This would have the dual benefit of speeding up trips between the heart of the West & East, as the current trip via the U2 is quite slow.

Yeah, so before I saw the S21 plans I wanted to crayon a southeast-northwest S-Bahn trunk with stops in Moabit. Zukunft-Mobilität does have the U5 extension, I’m just skeptical about it for the following reasons:

– There’s a big mismatch of demand from the east and west, so it’s fine to have some lines that only go center-to-east and don’t run through.
– The U5 extension would be exclusively about Moabit, since everything else gets equivalent S-Bahn service under S21.
– There’s no real point in keeping Tegel open – BER for all its faults is going to have faster trips to the center what with all the money that’s being wasted on direct rail connections. As soon as BER opens, the highest and best use for the area is housing and mixed-use development, and not an airport.

What you say about a line headed northeast is absolutely true, though; it could link with U3, and then U1 would have its 5-minute frequency restored west of Wittenbergplatz, which would interact well with plans to extend U1 west to Adenauerplatz. But figuring out a trunk route through the center is annoying. There’s a pretty direct route going under Leipziger Strasse, but that’s a street that’s so central that both the Stadtbahn and U2 avoided it to avoid street-level disruption. Then again, U5 is going under Unter den Linden so clearly there is recent precedent for this…

Berlin seems to be an example of the third line problem: namely, if you have a transit system that has two orthogonal trunk-lines (north-south and east-west, for instance), which Berlin has with the S-Bahn, then it is awkward to turn it into a Soviet triangle: firstly because you are inevitably going to under- and over-serve parts of the city by choosing either a NE-SW or NW-SE orientation (and foregoing the other one), and secondly because of the difficulty in creating a three-point hub, since the intersection between the first two lines will likely be overly dominant. London will be faced with a similar problem with Crossrail 2 (if we consider Thamselink + Crossrail 1 as forming a rudimentary S-Bahn system together), and neither city has any plans for a fourth line in even the long-term future.

The takeaway being that cities should, over the long term, decide to either adopt a Soviet triangle, in which case the first two lines should be slightly askew from each other so that the final system has roughly equal line spacing, or a four-line system covering the eight points of the compass. In the latter case, though, how do you envisage the geometry of central interchanging – assuming you avoid a four-way hub in the centre (which would be a nightmare for passenger circulation)? I find it hard to come up with a satisfying solution.

I don’t see any of that as a real problem. Few cities are perfectly symmetrical in shape or demand, so you build whichever of NE-SW or NW-SE has higher demand or covers the larger uncovered sector. As for London, the intersection between Thameslink and Crossrail is not overly dominant, in fact the intersection between Thameslink and Crossrail 2 would be more dominant. And London has in fact vaguely speculated on a fourth line (google “crossrail 3”).

As for a 4-line system, start with a Soviet triangle system and route the 4th line so that it intersects all 3 lines near the city center (hopefully at stations that were not already transfer stations, because as we know 3-line-stations tend to create load issues, but if this happens it’s not the end of the world).

In Berlin, the north-south line isn’t exactly north-south. At the northern end it goes north and fans out from there, but at the southern end it goes southwest (via the Wannsee Railway, used by S1), leaving a service gap in the southeast, served by the Görlitz Railway. In the northwest the only real service gap is to Tegel, but in general the S-Bahn lines going west are underused.

I’ve always thought they should build it, but it’s one of those too big/too small projects: too big to just be considered part of a regular infrastructure renewal program, and too small to be touted as a major investment in a new transport project. Berlin has quite a few of these missing links: Uhlandstraße-Halensee, Warschauer Straße-Frankfurter Tor, Ruhleben-Spandau, which will probably always be a medium-long-term priority, but always being shunted below more high-profile works (e.g. U5). Paris has the same thing with the 3bis/7bis merger.

Alon, what kind of a service plan would you envisage for the S-Bahn with the SW line in place? It seems hard to avoid reverse-branching entirely, or to turn the Ringbahn into a pure orbital line.

Can’t they use the American (or French?) method of announcing multiple lines at once in different parts of the city as a package?

The service I envision is,

– North-South Tunnel: Wannsee-Oranienburg, Bernau-Teltow/Blankenfelde; S8/S85 are to cease operations, with the Outer Ring parts of S8 possibly run as a short tram-train, similarly to the Parisian plans for the Grande Ceinture. Peak takt should be 5 minutes for each of these two branches (and 10 minutes to each of Teltow and Blankenfelde) but presumably a lot of trains would short-turn.

– Stadtbahn: each of the four eastern branches – Wartenberg, Ahrensfelde, Strausberg North, Erkner – gets a 10-minute peak takt, again with potential short-turns. Going west, I presume half the trains turn at Westkreuz, and then Potsdam and Spandau get 10-minute takts, but I don’t know what the pairings should be.

– S21-Görlitzer Bahn: going southeast, four branches – Spindlersfeld, Schönefeld, BER, Königs Wusterhausen – get 10-minute peak takt each. If it’s too much service, then reduce Spindlersfeld to a tram with a transfer at Schöneweide, which also conveniently avoids a flat junction. Going northwest, there are three branches with 10-minute takt each: Siemensbahn, the Tegel branch I drew, and the other Tegel branch taking over S25. If Spindlersfeld isn’t cut from the mainline then a fourth northern branch could go east to Gesundbrunnen as is planned for S15.

The S-Bahn tracks aren’t set up to branch to serve BER; rather, they serve the existing Schönefeld station westbound, then curve back to the east to terminate in the BER station. This is visible on OpenStreetMap, where S-Bahn tracks are shown differently than the mainline tracks: https://www.openstreetmap.org/#map=13/52.3812/13.5070

So the three southeastern branches could be Spindlersfeld, Schönefeld/BER, and Königs Wusterhausen.

What do you do with Köllnische Heide? It’s a useful enough connection in the network. You could run the Ringbahn as a teacup-line to Spindlersfeld, like London’s Circle line, but there’s nowhere to turn trains at Neukölln.

Under the southern option, it still gets served, it just loses its through-service to Neukölln. Under the northern one, it may be reduced to a shuttle.

I don’t think London’s teacup route is a great way of running trains; it works for London because the Circle line has flat junctions galore, and is both conceptually and operationally really just supplementary service on the Met and District lines.

it’s me who wrote the article on the blog „Zukunft Mobilität“ whose webmaster has informed me about your post. Nice to see the proposal of S21 South East also being proposed and discussed here! I have some comments to your article and the discussion:

> Since reunification, Germany has made an effort to move the Berlin central business district back to the historic center, and S21 is to reinforce that, serving the western end of Mitte.

Hauptbahnhof [Central Station] and Potsdamer Platz are actually located on the historic city borders during 18th century and until now haven‘t become a real CBD yet as they are more popular among tourists than among normal residents. Between the two stations there are a lot of governmental and parliamentary institutions which also don’t cause much demand as an S-Bahn station at Bundestag will not be built. North of Hauptbahnhof, the new Europacity quarter is being developed now, but a station at Perleberger Brücke won’t be opened in the first phase and would be quite complicated because of the two curves. Gleisdreieck is an interchange station in the middle of nowhere, surrounded by railway areas and a new park. Most long-distance trains also stop at Südkreuz and some also at Gesundbrunnenwhich are both already connected to the existing Nordsüdbahn.

> The new four-track line has ample capacity for additional S-Bahn traffic, but nonetheless it hosts no S-Bahn trains in regular service.

The reason is simple: Mainline tracks are electrified with 15 kV 16,7 Hz AC and S-Bahn tracks with 750 V DC. There were unofficial proposals for express S-Bahn services operated with AC on the mainline tracks (see e.g. http://www.sx-bahn.de) or even hybrid systems changing to DC to continue on suburban tracks, but such a service with special branding and dense intervals hasn’t been introduced yet. However, there were temporary shuttle services with Western German S-Bahn trains (AC) during FIFA World Cup 2006 as well as 2009/2010 during the major service restrictions of Berlin S-Bahn (DC).

> The segment from Hauptbahnhof to Potsdamer Platz, which contrary to the map is to be nonstop, is in early stages of construction, and Wikipedia says it is expected to open in 2023.

2023 is not realistic, as the construction around Bundestag [German parliament] is very complicated, so that actually one tunnel will pass in front and the other tunnel will pass in the back of the building.

> Farther south of Potsdamer Platz is still not under construction, and frankly should not be built as is.

As you also write later, there is actually nothing to say against a new section via Gleisdreieck. After the third phase is built, the southeastern extension of S21 could just take over the tunnel to Anhalter Bahnhof station which wouldn’t be used for trains toward the south anymore. Apart from this branch the only tunnel which would have to be adapted is the junction north of Potsdamer Platz where the tracks from/to Hauptbahnhof would become the tracks from/to Gesundbrunnen and vice versa.

> It specifically criticizes S21 for duplicating existing infrastructure, and proposes an extension to the southeast, mentioning that there were plans to that effect in the 1930s.

As far as I know, the plans for the 1930s included an S-Bahn connection between the former long-distance terminal stations Görlitzer Bahnhof and Anhalter Bahnhof as well as a branch to Potsdamer Platz (that’s why the S-Bahn stations Anhalter Bahnhof and Potsdamer Platz were built with 4 tracks), but not further northwest towards the long-distance Lehrter Bahnhof and Hamburger Bahnhof located next to the current Hauptbahnhof (see e.g. de.wikipedia.org/wiki/Ost-West-S-Bahn).

> The dashed lines denote under-construction lines, including S21 to Potsdamer Platz, the 4.5-kilometer Siemens Railway to the northwest, and the U5-U55 connection.

Siemens Railway is not yet under construction, but will probably be reopened in a few years as part of a deal with the company Siemens, which used to have its headquarter in Siemensstadt before World War II and plans to revive the quarter as a new city research campus and residential area.

> Two branches are depicted toward Tegel, one toward airport grounds to be redeveloped, and one going farther north taking over S25; there are already discussions of a rapid transit line to Tegel, branching off of U6, but this option does not force the outer parts of U6 to contend with reduced frequency.

Connecting S25 to the new instead of the old North-South tunnel is problematic as it would mean that the Alt-Reinickendorf and Karl-Bonhoeffer-Nervenklinik (interchange with U8 e.g. to Alexanderplatz!) would have to be closed. Instead, this line will get a second track to allow 10-minute headway to Tegel and perhaps even regional railways to the Prignitz. To avoid confusion: The existing of U6 goes to Alt-Tegel in the existing quarter of Tegel, the branch would go to the area of the current Tegel Airport.

> All reverse-branching would cease: the various branches on the Görlitz Railway, including the existing ones as well as an under-construction one to the airport-to-be, would feed into the S-Bahn trunk, rather than to the Ring or the Stadtbahn. The removal of S25 from the North-South Tunnel would create space for the S8 and S85 services in Pankow to use the North-South Tunnel instead of diverting to the Ring and Görlitz Railway.

Introducing S8 and S85 into the Nordsüdbahn would simplify operations but might cause protests among passengers which are used to have a connection to the eastern Ring railway since the construction of the Berlin Wall when this curve (called after the then Eastern German head of state Ulbricht) was build to connect the northeastern branches with the remaining East Berlin S-Bahn network as the North – South tunnel was part of the West Berlin S-Bahn network, so it should be left open as a branch of the Ring railway at least to Pankow, but ideally to the existing S8 along the Outer Ring railway, as a tram-train like on the Grande Ceinture in Paris sounds like a drawback.

> It also has a peculiarity in that S75 trains only run east of Ostbahnhof; I can’t tell if there’s a problem with track capacity or demand mismatch, but if it’s the former then it’s strange since peak S-Bahn traffic on the Stadtbahn is only 18 trains per hour (Munich achieves 30 through its central tunnel, with much higher crowding levels), and if it’s the latter then it’s again strange – why not run through to Westkreuz like S5?

I agree that there would be free capacity as there are gaps of up to 4-5 minutes in the timetable. However, this capacity could also be used by every other S3 train also ending at Ostbahnhof. An explanation why S75 is selected to always start at Ostbahnhof is that there is also the more frequent and (geographically) more direct tram line M4 from Alexanderplatz to Hohenschönhausen. Indeed, the Munich S-Bahn trunk route has a higher capacity because of automatic train operation using a German system called LZB (Linienzugbeeinflussung).

> S21 is another of these little mistakes. It’s a prestige project on the heels of the construction of Hauptbahnhof, rather than a solution to a transportation need. There are six north-south tracks through Berlin between the S-Bahn and the mainline and they’re not anywhere near capacity; the mind boggles at why anyone would add seventh and eighth tracks before adding fifth and sixth east-west tracks.

You didn‘t even mention the major bottleneck resulting from the triangle between Hauptbahnhof, Westhafen and Wedding. Note that that all three stations don’t have a third track which could be used if two trains from different directions to the same direction enter the station at the same time and a similar problem could also arise when S15 changes between the Ring and the North-South railways at Gesundbrunnen. Even worse, Wedding and Westhafen are located on the Ring railway, which has a highly critical stability anyway. In the first phase, there will only be a reduced shuttle service from Gesundbrunnen to a temporary terminus at Hauptbahnhof, and even later the lines serving the S21 will only have unattractive intervals of 20 minutes each. Furthermore, unless the third phase via Gleisdreieck is open, the bottleneck at Anhalter Bahnhof means that there is no additional capacity compared to the old North-South tunnel.

> Zukunft-Mobilität does have the U5 extension, I’m just skeptical about it for the following reasons:
– There’s a big mismatch of demand from the east and west, so it’s fine to have some lines that only go center-to-east and don’t run through.
– The U5 extension would be exclusively about Moabit, since everything else gets equivalent S-Bahn service under S21.
– There’s no real point in keeping Tegel open – BER for all its faults is going to have faster trips to the center what with all the money that’s being wasted on direct rail connections. As soon as BER opens, the highest and best use for the area is housing and mixed-use development, and not an airport.

U5 has some benefits as it would provide a fast and reliable connection from Jungfernheide (interchange to U7 at the same level!) and Turmstraße in Moabit to Hauptbahnhof, Brandenburger Tour Unter den Linden and the even more important public transport hub Alexanderplatz. The good cost-benefit ratio which was calculated for the U5 extension included the section to Turmstraße! However, due to the parallel construction of S21 as well as tram M10 to Turmstraße it has been postponed indefinitely by the city government and I agree that it shouldn‘t have highest priority. Unforunately the current plans for the Tegel area don’t envisage real mixed-use development, as housing is mainly restricted to the eastern border next to the existing U6, while the central parts around the old terminal would be used for industry and higher education, so the trains on the new U6 branch would be empty at the weekend.

> What you say about a line headed northeast is absolutely true, though; it could link with U3, and then U1 would have its 5-minute frequency restored west of Wittenbergplatz, which would interact well with plans to extend U1 west to Adenauerplatz. But figuring out a trunk route through the center is annoying. There’s a pretty direct route going under Leipziger Strasse, but that’s a street that’s so central that both the Stadtbahn and U2 avoided it to avoid street-level disruption. Then again, U5 is going under Unter den Linden so clearly there is recent precedent for this…

The reason why U2 doesn’t have a straight alignment along Leipziger Straße is that the private tramway companies opposed it 100 years ago! In the meanwhile, the trams were abandoned, but now the city government wants to reintroduce them, although they will probably often be stuck in the frequent traffic jams on Leipziger Straße. During the construction of the mainline North-South tunnel, a metro station for a future U3/U10 was built at Potsdamer Platz, but this will be used neither for the metro nor for an underground tram in the near future. Instead of a separate route for U3, I could even imagine U2 and U3 sharing tracks between Wittenbergplatz and Nollendorfplatz, but this would be close to capacity, so that automation would probably be required for a stable service.

> It’s a gap, but what likely journeys does closing that gap enable? Freie Universität to Wannsee? Demand is inbound from both of these stops.

An extension of the U3 to Mexikoplatz would make more sense combined with an extension of the S1 to Potsdam, because journeys from Dahlem to Potsdam would then be possible with just one interchange compared to 3-4 interchanges now, with benefits particularly for students studying either at Freie Universität in Dahlem or at Universität Potsdam and living at the other place, respectively. Perhaps also some passengers from U3 to Zehlendorf district center would change to S1 at Mexikoplatz instead of the buses at Oskar-Helene-Heim.

> If Spindlersfeld isn’t cut from the mainline then a fourth northern branch could go east to Gesundbrunnen as is planned for S15.
I think that such a branch would hardly have advantages compared to the existing Nordsüdbahn and that the disadvantages of the mixed operation with the Ringbahn overweigh.
> What do you do with Köllnische Heide? It’s a useful enough connection in the network. You could run the Ringbahn as a teacup-line to Spindlersfeld, like London’s Circle line, but there’s nowhere to turn trains at Neukölln. > Under the southern option, it still gets served, it just loses its through-service to Neukölln. Under the northern one, it may be reduced to a shuttle.

Conversely, with the southern option, the northern connection via Plänterwald could still be used by S9 running on its own tracks from Warschauer Straße via Treptower Park and Baumschulenweg to BER in order to retain a connection of the Stadtbahn east-west line with the Adlershof science campus of Humboldt University, which has its main campus along Stadtbahn, and the new airport, although this would contradict with the aim to use the capacity on the Stadtbahn for other lines. I slightly prefer to keep the S21 network connected to the Ring network at Westhafen/Wedding as well as Neukölln/Treptower Park and just separate it from the Stadtbahn and Nordsüdbahn.

I‘ve seen that you’re also a mathematician interested in public transport and now also living in Berlin!

This takes quite a bit of detailed knowledge. Anyway: in watching a video of the night train from Moscow I noticed it ran over the Goerlitz Line in Berlin, but had to be shunted through an extensive maze of forward-then-reverse directional permutations to get to the new Main Station. It probably consumed an hour in doing this. It seemed pretty obvious how this could be rectified (though I don’t remember now having not saved my line drawing with surmised curves) and this would be essential if competitive service is to be had. While Berlin-Moscow may be a stretch Berlin-Warsaw is something that should be developed aggressively now I would say – and although (not-necessarily) the customary route passes through Warsaw. I do love the idea of Berlin-Moscow and Berlin-Rome greatest train on earth contenders for sure, but the state of passability is not anywhere near making that feasible, including the new Leipzig Bypass. (pardon the criticism on that)

Berlin-Warsaw does have something like 6 or 7 trains a day (and 1 a day to Gdańsk/Gdynia). They normally run on the Silesian line (aka Niederschlesisch-Märkische Eisenbahn): the long-distance AC-electrified lines next to Stadtbahn, stopping at Ostbahnhof and passing Ostkreuz and Erkner on the way to Frankfurt (Oder).

Not sure why the Moscow train would run over the Goerlitz line, other than they’ve been doing construction on the Silesian/Frankfurt line so maybe it was detoured.

Checking now, the Berlin-Warsaw trains currently have 4 direct runs a day, and a fifth is the train to Gdynia with change for Warsaw in Poznań. Having taken them, they are pretty alright and seem decently well used. Travel time is currently around 6-7 hours though this is because the line through Konin between Poznań and Warsaw is being rebuilt, I seem to recall it was under 6 hours otherwise. Getting them to a regular every-2-hours schedule would be nice, maybe when the construction is done?

Other improvements would probably be line speed… you’d probably need 200 km/h to get it under 4 hours. The line is currently max 160 km/h but slower parts remain and the average speed within is apparently 105 km/h both on German and Polish sides.